Electronic – Is it right to apply superposition theorem to the RC circuit with non-zero initial capacitor voltage

It's like the "paradox" of the immovable object meeting the irresistible force. In reality, neither can exist.

A real power source will have some impedance. A real capacitor will have some impedance. Real wires connecting them have resistance and inductance.

So in reality when you slap a fairly 'stiff' power source across a fairly good real capacitor there's a spark and the capacitor charges through those series resistances with some ringing and stuff due to the inductances. Ignoring the inductances, the voltage difference would simply divide in ratio to the internal resistance of the capacitor and the internal resistance of the power supply and the wire resistance.

To answer your specific question: If the capacitor and voltage source and wires are ideal, you have a mathematical problem, like division by zero. It's of no consequence in the real world- it just illustrates that the ideal models of the power source and the capacitor and wires are insufficiently accurate to describe their real-world behavior. Modelling any one of those as a real part with real resistance (and inductance) will make the mathematical problem go away, but it won't likely give you an accurate indication of what is actually happening.

For example, if the wire (or the capacitor or the power supply) had 10m\$\Omega\$ resistance, and there is a 10V difference, you could predict you'd see 1000A (which is very high, but not infinity) and the capacitor would charge very quickly. In reality that isn't likely going to happen because of other non-ideal factors. If the 10m\$\Omega\$ was modeled as in the power supply, the power supply voltage would drop. If the 10m\$\Omega\$ was modeled as in the capacitor, the voltage would suddenly appear across the capacitor terminals. If the 10m\$\Omega\$ was modeled as in the wire, the voltage would appear across the wire. But none of those is very realistic.

If you modeled as a circuit with no resistance at all and a tiny bit of inductance (even superconducting wires of any length have inductance) then a simple mathematical model would predict ringing that would persist forever, energy sloshing back and forth between the inductance and capacitance at an angular frequency of \$\omega_0 = {1 \over \sqrt{LC}}\$.

Since the system has initial conditions, the equations are differential equations. With your example, there are three 2nd order differential equations.

Since the equations are linear, the voltage sources (inputs) will always appear as the inhomogeneous terms (aka, input functions, forcing functions). When solved, each equation will produce a particular solution and a homogeneous solution with constant(s). With your example, 3 pairs of particular solutions and homogeneous solutions each with 2 integrating constants.

So it appears that there are two many constants to be determined by the initial conditions, but the homogeneous solutions will be linearly dependent, so the number of constants collapse back to just one set. In your example, the 6 constants collapse back to 2. Knowing this, you could just take the 3 particular solutions and any one of the homogeneous solutions, add them together and that is the general solution for the system. Then fix the 2 constants with the initial conditions.